Ideas are like rabbits. You get a couple and learn how to handle them, and pretty soon you have a dozen.
A vigorous five-mile walk will do more good for an unhappy but otherwise healthy adult than all the medicine and psychology in the world.
Perpetual optimism is a force multiplier.
The voice of conscience is so delicate that it is easy to stifle it; but it is also so clear that it is impossible to mistake it.
Destroying species is like tearing pages out of an unread book, written in a language humans hardly know how to read, about the place where they live.
Age is no guarantee of maturity.
The greatest compliment that was ever paid to me was when one asked me what I thought and attended to my answer.
It is as easy to dream a book as it is hard to write one.
What you do speaks so loud that I cannot hear what you say.
A thinker sees his own actions as experiments and questions—as attempts to find out something. Success and failure are for him answers above all.